In biological sewage treatment plants, the biological sewage treatment process is carried out aerobically and anaerobically using microorganisms to eliminate the organic impurities in the sewage. In the metabolic processes which take place, the microorganisms proliferate in large numbers.
In practice, the so-called activated sludge process is predominantly used for the aerobic and/or anaerobic treatment process using the continuous-flow principle. In continuous-flow operation, sewage and activated sludge are fed to the activated-sludge tank where, in the aerobic or anaerobic process, the biochemical processes take place. The oxygen may be supplied in such a way that, at the same time, high turbulence is generated in the activated-sludge tank, ensuring optimal mixing of sewage and activated sludge. The mixture of sewage and activated sludge flows from the activated-sludge tank into intermediate or final settling tanks in which the activated sludge separates from the purified sewage. To maintain as high a concentration of activated sludge as possible in the activated-sludge tank, most of the activated sludge deposited in the intermediate or final settling tanks is recycled to do more biological treatment work. Only the surplus activated sludge formed by proliferation of the microorganisms is removed from the system and fed to the clarified sludge disposal stage.
Accordingly, most of the activated sludge is in a state of permanent circulation. However, the separation of sewage and activated sludge in the final settling tanks is difficult because, in many cases, the activated sludge is particularly light and, because of this, only settles very slowly or incompletely. However, a high degree of settling and the substantially complete recycling of the activated sludge are essential for ensuring the necessary degree of purification and for remaining within legally stipulated limits.
It is standard practice to base the dimensions of the intermediate and final settling tanks on retention times of from 2 to 4 hours. However, where the tank volume is predetermined by the retention time, it is only possible to achieve a high degree of efficiency when the dimensions of the final settling tanks are also optimally selected for the settling process of the activated sludge.
Determining factors so far as the dimensions are concerned are the length, width or depth, diameter and volume of the final settling tank. The dimensions are characterized by the load per unit area per hour (m.sup.3 sewage/m.sup.2 surface.h). The load per unit area of the final settling tanks must always be lower than the settling rate of the activated sludge. However, the permitted limit for the load per unit area of the final settling tanks is ultimately determined by the condition and quantity of the sludge introduced with the sewage.
Settling behavior is characterized by the sludge index (Isv) which indicates how large the volume (V.sub.s) of 1 g of dry sludge matter (TS.sub.R) is after a settling time of 30 minutes. ##EQU1## With properly dimensioned tanks, it is generally possible to obtain a good settling effect for a sludge index of &lt;100 ml/g. If, however, the settling process is impeded by high solids loads and, in particular, by the formation of bulking sludge, the activated sludge floats and drifts.
Settling processes are seriously impeded, beginning at a sludge index of &gt;150 ml/g. There are various known causes for this increase, including, for example, the inclusion of materials of light specific gravity (such as fats) in the activated sludge, buoyancy through adhering gas bubbles (particularly in denitrification processes) and especially the formation of bulking sludge through filament-like organisms which proliferate in relatively large numbers.
This phenomenon is particularly serious in that it increases the settable materials in the effluent of the treatment plant, which can exceed by many times the legally stipulated maximum limit of 0.5 ml/l, and thus, deprives the system of biologically active sludge which seriously reduces the efficiency of treatment.
To improve the settling behavior of activated sludges and to eliminate bulking sludge, the literature (cf. Lehrand Handbuch der Abwassertechnik, Vol. II, 2nd Edition, and Korrespondenz Abwasser No. 4, 1985) describes such measures as, for example, damaging the filament-like microorganisms by chlorine or hydrogen superoxide, process modifications, increasing the weight of the activated sludge by preclarified sludge, adding lime and/or iron or aluminium salts.
All these measures are attended by the disadvantage that they are only partly successful, if at all, or are only effective after prolonged treatment times or involve very considerable expense or give rise to disadvantages in the subsequent disposal of the clarified sludge.
The use of alkaline pretreated magnetite particles for the removal of discoloration and turbidity from river water is described in L. O. Kolarik, Water Research, Vol. 17, p. 141-147 (1984). The magnetite is regenerated by acidifying (emission of absorbed particles) and subsequent alkali treatment (magnetite particles become again positively charged and active). Untreated magnetite proved to be practically without effect for the river water treatment.
Similar water purification processes are described by C. deLatour and H. K. Holm in the Journal of American Waterworks Association, June 1977, p. 325-327. Other water purification processes with magnetite relate to the removal of algae through absorption with magnetite in the presence of ferric chloride (R. Yadidia et al., Enviromental Science and Technology, Vol. 11, No. 9, p. 913-916, 1977).
It has now surprisingly been found that the disadvantages attending the biological treatment of sewage can be avoided by using magnetically separable materials which, as carrier materials, provide for rapid settling of the activated sludge as a whole. In addition, it is possible by addition of these magnetically separable materials to obtain a distinctly higher concentration of activated sludge and, hence, higher throughputs in the activated sludge tanks and the final settling tanks or, by applying a magnetic field, to obtain excellent separation or, by magnetic separation, to separate off the activated sludge without final settling tanks.